INORGANIC PHOSPHATE PEAK SPLITTING DURING EXERCISE IN
HUMAN SKELETAL MUSCLES – WHAT DOES IT MEAN?
P. Kulinowski1,3, J. Zapart-Bukowska2, A. Jasiński1, and J.A. śołądź2
1
Department of Magnetic Resonance Imaging, Institute of Nuclear Physics Polish Academy of
Sciences, Kraków, Poland;
2
Department of Physiology and Biochemistry, University School of Physical Education,
Kraków, Poland;
3
Institute of Engineering and Technology, Pedagogical University, Kraków, Poland
Introduction
Inorganic phosphate (Pi) belongs to the muscle metabolites measured by the means of
31
P-MRS. Normally, single peak of this metabolite is located in the spectrum near 5 ppm.
It was reported that in some experimental conditions, especially during fatiguing physical
exercise the peak of the Pi in the working muscle can be splitted into two distinguish separate
peaks [1-8]. The origin of the appearance of this splitting as well as its physiological
significance is still unclear. In the present study we attempt to determine the changes
in muscle metabolites in human calf muscle during fatiguing plantar-flexion exercise with
special focus on the possible appearance of the inorganic phosphate splitting.
Subjects and Methods
Twenty two healthy young men age (mean ± S.E.) 23.4 ± 0.7 years old, height 184.0 ± 1.5
cm, body mass 74.6 ± 1.5 kg, BMI 22.0 ± 0.3 volunteered for this study. The exercise
protocol started with 5 minutes seating in rest, followed by performing right leg plantar
flexion exercise with frequency of 60 cycles per minute – until fatigue. The resisting force
amounted to about 50% of the maximal voluntary contraction force of this muscle group,
determined for each subject. After the end of exercise the rest period was started and last until
at least 30 minutes after termination of exercise. Muscle metabolites were measured in the
calf muscles at rest, during the exercise and during the recovery by means of 31P-MRS, using
a 4.7T superconducting magnet (Bruker) and MARAN DRX console. After the preprocessing
MR data sets were analyzed in the time-domain using the “JMRUI v.3.0” software package.
Intracellular pH was calculated applying the Henderson-Hasselbalch equation.
Results
Inorganic phosphate peak splitting was observed in 7 of 22 subjects. Average time
of appearance of the splitting was 1.81 ± 0.49 minutes after starting the fatiguing exercise
lasting 5.32 ± 1.40 minutes. The splitting was present until the end of the exercise but
it disappeared during the recovery period. Example spectra stack-plot for one subject
is present in Fig 1.
Intracellular pH of the muscle at rest for all subjects (n=22) was 7.07 ± 0.01. Basing on the
chemical shifts of the two Pi peaks (Pi1 and Pi2) two pH values were calculated, namely pHlow
and pHhigh. Average pHlow at the end of exercise was 6.59 ± 0.05 while pHhigh was
7.01 ± 0.02. For the same group when fitting Pi as a one peak the mean pH at the end of the
exercise was 6.73 ± 0.05. For the group without splitting the average pH at the end
of the exercise was 6.72 ± 0.05.
Discussion & conlusions
According to Yoshida and Watari [4,5] and Mizuno et al. [6] the Pi peaks can be attributed
to two types of muscle fibers recruited during exercise. The peak associated with low
pH represents recruitment of type II muscle fibers (glycolytic) whereas the peak with high pH
represents recruitment of type I muscle fibers (oxidative). This suggestion is in accordance
with Park et al. [1].
Fig 1. Stackplot of 31P MRS spectra zoomed to Pi – PCr region for one subject with Pi
splitting. Single Pi peak (timecourse of the peak position marked with yellow line)
splits to Pi1 with high pH (red line) and Pi2 with low pH (green line). a – start
of exercise, b – onset of the Pi splitting, c – end of exercise.
According to Rossiter et al. [8] the Pi peak splitting reflects rather heterogeneous
contribution to force production in various regions of the working muscle. This suggestion
is in accordance with Jeneson et al. [3].
Another explanation for the appearance of the Pi peak splitting during muscle exercise can
be varied metabolic stability in some regions of the muscle (for discussion of this point see
Korzeniewski [9], Zoladz et al. [10]), most likely related to varied muscle fiber composition
and their involvement in force production.
Acknowledgment: This study was supported by funding from the University School
of Physical Education (AWF Kraków) for the statutory research 2007 and from the Institute
of Nuclear Physics (IFJ PAN Kraków).
References:
[1] Park JH et al., Proc Natl Acad Sci USA, 1987, 84: 8976-8980.
[2] Vandenborne K et al., Proc Natl Acad Sci USA, 1991, 88: 5714-5718.
[3] Jeneson JA et al., Am J Physiol, 1992, 263: C357-364.
[4] Yoshida T, Watari H, Eur J Appl Physiol Occup Physiol, 1993, 67: 274-278.
[5] Yoshida T, Watari H, Eur J Appl Physiol Occup Physiol, 1994, 69: 465-473.
[6] Mizuno M, Secher NH, Quistorff B, J Appl Physiol, 1994, 76: 531-538.
[7] Houtman CJ et al., J Appl Physiol, 2001, 91: 191-200.
[8] Rossiter HB et al., J Appl Physiol, 2002, 93: 2059-2069.
[9] Korzeniewski B, Biochem J, 2003, 375: 799-804.
[10] Zoladz JA, Korzeniewski B, Grassi B, J Physiol Pharmacol, 2006, 57 Suppl 10: 67-84.